1. Field of the Invention
The present invention relates to a UV-fixable thermal recording apparatus for forming an image using UV-fixable thermal recording paper on which a full color image can be formed by heating. This apparatus may be used for a color printer, a video printer, a color facsimile or a like device which serves as an output device of a personal computer or a word processor.
2. Description of the Prior Art
UV-fixable color thermal recording paper is one on which respective colors can be selectively fixed by light rays, such as ultraviolet rays or electromagnetic waves. The developed color density of the paper varies depending on the amount of heat applied, the minimum amount of heat differs depending on which of the colors is to be developed and the particular wavelength. It has the advantages that full color recording can be made thereon without using ink, as compared with conventional ink-jet recording or thermal transfer recording.
UV-fixable color thermal recording paper is disclosed in Japanese Patent Laid-Open No. 40192/1986, for example. The UV-fixable thermal recording paper is constructed by laminating a yellow color forming layer, a magenta color forming layer and a cyan color forming layer in order from the top on a base film. The minimum amounts of heat respectively required to develop yellow, magenta and cyan colors are increased in this order, and the yellow color forming layer and the magenta color forming layer are irradiated with ultraviolet rays having different particular wavelength ranges so that they are fixed. On the other hand, the cyan color forming layer is not fixed.
In the case of recording on the UV-fixable thermal recording paper, at the time of developing a yellow color, an amount of heat which is less than the minimum amounts of heat respectively required to develop magenta and cyan colors are applied, whereby magenta and cyan colors are not developed, and only a yellow color can be developed. After developing a yellow color, only the yellow color forming layer is fixed by a fixing lamp for the yellow color.
At the time of developing a magenta color, an amount of heat which is less than the minimum amount of heat required to develop a cyan color is applied, whereby a cyan color is not developed. Since the yellow color forming layer has already been fixed, a yellow color is not developed again. After developing a magenta color, only the magenta color forming layer is fixed by a fixing lamp for the magenta color.
At the time of developing a cyan color, the yellow and magenta color forming layers have already been fixed, whereby yellow and magenta colors are not developed again, and only a cyan color can be developed.
A multihead type recording apparatus using UV-fixable color thermal recording paper is disclosed in Japanese Patent Laid-Open No. 24233/1993, for example. FIG. 25 of the present application is a cross-sectional view showing this conventional recording apparatus.
The recording apparatus comprises heating resistors 51a, 51b and 51c in three rows, and notches 56a and 56b respectively provided between the adjacent heating resistors 51a, 51b and 51c. The heating temperatures of the heating resistors 51a, 51b and 51c in three rows are set to decrease in the order in which the heating resistors come into contact with thermal recording paper 60, and the heating resistors 51a, 51b and 51c are respectively used for yellow, magenta and cyan colors.
Individual electrodes 52a and 52c and common electrodes 53a, 53b and 53c are respectively connected to the heating resistors 51a, 51b and 51c, and the individual electrodes 52a and 52c are connected to a head driving circuit 57. The illustration of the individual electrode and the like connected to the heating resistor 51b is omitted.
A fixing lamp for a yellow color 54a is arranged between the heating resistor for a yellow color 51a and the heating resistor for magenta color 51b, and a fixing lamp for magenta color 54b is arranged between the heating resistor for a magenta color 51b and the heating resistor for cyan color 51c.
Exposure of the UV light from lamps 54 to the thermal recording paper 60 is made through the notches 56a and 56b which are provided in a head 50. Lenses 58a, 58b, 59a and 59b are provided in order to efficiently collect light from the fixing lamps 54a and 54b and direct it onto the thermal recording paper 60. A shading member 55 is further provided in order to prevent light from being irradiated onto portions other than the corresponding portions.
Referring still to FIG. 25, description is now made of the recording operation of the recording apparatus. The thermal recording paper 60 is conveyed with it being interposed between the head 50 and a platen roller (not shown). A yellow color forming layer is developed by the heating resistor for a yellow color 51a, and is fixed by the fixing lamp for the yellow color 54a. A magenta color forming layer is then developed by the heating resistor for the magenta color 51b, and is fixed by the fixing lamp for the magenta color 54b. A cyan color forming layer is finally developed by the heating resistor for the cyan color 51c.
A serial thermal head type recording apparatus using UV-fixable color thermal recording paper is disclosed in Japanese Patent Laid-Open No. 124352/1993, for example. FIG. 26 is a perspective view showing the construction of this recording apparatus.
As shown in FIG. 26a fixing lamp for a yellow color 63a and a fixing lamp for a magenta color 63b are arranged with a thermal head 62 interposed therebetween in a serial head section 61. The serial head section 61 is so arranged as to be movable back and forth in the width direction of thermal recording paper 60 by the rotation of a timing belt 64. The longitudinal thermal recording paper 60 is sent to the print position in which the serial head section 61 exists by a conveying roller 65a positioned on the upstream side, a conveying roller 65b positioned on the downstream side, a pinch roller 66a positioned on the upstream side and a pinch roller 66b positioned on the downstream side. The thermal recording paper 60 is cut by a cutter 67 when it is moved by a predetermined length.
Referring again to FIG. 26, the recording operation of the recording apparatus will be described.
Printing is done while the serial head section 61 is moved in a direction perpendicular to the direction of delivery of the thermal recording paper 60 (direction of the arrow) in synchronism with the conveyance of the thermal recording paper 60. The serial head section 61 is moved in such a direction of progress that the fixing lamp for a yellow color 63a is in the rearward position of travel at the time of printing of a yellow color, while the fixing lamp for a magenta color 63b is in the rearward position at the time of printing of a magenta color. Yellow, magenta and cyan colors are successively printed for each line.
In recording using the UV-fixable color thermal recording paper, when an area where printing has not yet been done is exposed by the fixing lamps, no colors are developed even if it is then heated for printing. Therefore, a device in which the fixing lamps are enclosed by a skirt or shield made of rubber or the like to intercept light has been proposed.
The problems with the above-described UV-fixable thermal recording apparatus, will now be described for the case of a serial thermal head in which a row of heating resistors and a fixing lamp are adjacent to each other.
FIGS. 27A to 27I illustrate the positional relationship between the length of a row of heating resistors 70 (the width of recording) in the serial thermal head and the width of exposure 71 in the direction of paper delivery in a case where the fixing lamp is enclosed by a shield member.
FIGS. 27A, 27B and 27C illustrate a case where the position DD of an end on the upstream side in the forward direction of paper delivery of the row of heating resistors 70 is on the upstream side of the position BB of an end on the upstream side of the exposure area 71 in this direction. In this case, in FIGS. 27A, 27B and 27C, a portion between line segments BB and DD has not been fixed yet and is developed at the time of printing colors in the succeeding line.
FIGS. 27D, 27E and 27F illustrate a case where the position BB of an end on the upstream side in the forward direction of paper delivery of the row of heating resistors 70 is on the downstream side of the position DD of an end on the upstream side of the exposure area 71 in this direction. In this case, even a part of an area which has not been developed yet by heating in the succeeding line is exposed to be fixed. In FIGS. 27D, 27E and 27F, a portion between line segments BB and DD has been exposed before being developed, and is not developed at the time of printing colors in the succeeding line.
FIGS. 27G, 27H and 27I illustrate a case where the position BB of an end on the upstream side in the forward direction of paper delivery of the row of heating resistors 70 is the same as the position DD of an end on the upstream side of the exposure 71 area in this direction. In this case, the above-described problems do not occur. However, it is difficult to realize the positional relationship between the row of heating resistors and an exposed area with high precision. In addition, light also leaks into a peripheral end of the exposed area, whereby it is difficult to strictly realize the positional relationship. When the positional relationship is disturbed, there occurs such degradation of the print quality that stripes of the pitch between the widths of recording are formed from the above-described reasons.
On the other hand, as the conventional fixing lamp, a fluorescent lamp in the shape of a tube for emitting ultraviolet rays is used. The fluorescent lamp generally has counter electrodes at both ends in the longitudinal direction of the tube, whereby the quantity of light is small so that uniform light quantity distribution is not obtained in the vicinity of the electrodes. Therefore, fixing becomes nonuniform. For example, additional color development occurs by heat for developing the succeeding color in the vicinity of the electrodes where fixing is insufficient, thereby degrading the image quality. If in order to solve this, the intensity of light emission is increased as a whole, a middle portion of the paper will be exposed more than necessary. Although the peak wavelengths differ in spectral sensitivity differ in a case where a yellow color forming layer is fixed and where a magenta color forming layer is fixed, each spectral sensitivity has broad distribution. As a result, if the intensity of exposure of a yellow color forming layer first made is too high, a magenta color forming layer to be subsequently developed is prevented from being developed, thereby degrading color reproducibility.
Furthermore, in order to solve the decrease in the color reproducibility, if the length of the fluorescent lamp is larger than the width of recording so as not to use the vicinity of the electrodes where the quantity of light is small, the recording apparatus is increased in size. Further, light is unnecessarily emitted, thereby increasing the power consumption and the size of the power supply.
Additionally, it is desirable for thermal recording by the thermal head and fixing by the fixing lamp to be performed at the same speed in order to increase the recording speed. Further, the exposure conditions must be changed depending on the recording speeds of various specifications. Factors for determining a necessary quantity of light are the light intensity, the exposure area, and the moving speed. Since in the conventional fixing lamp, light quantity distribution is not constant, light intensity distribution is not constant. Therefore, a combination of an output of the fluorescent lamp and the exposure area for satisfying the requirements determined moving speed is at a particular by trial and error.